Present techniques for providing surgical access to the heart and coronary arteries are highly invasive. Cardiac surgery is conventionally performed via a median sternotomy, a procedure in which the patient's skin is incised at the midline overlying the chest, and in which the sternum is divided longitudinally along its entire length. Access may be gained to the coronary arteries for bypass surgery, or to the heart valves for valve replacement, by forming an incision in the pericardium, the fibrous sac enclosing the heart. The cut edges of the sternum are retracted with metal retractors, exposing a large cavity to allow surgery to be performed on the heart.
Access to the heart for certain procedures may be gained using a subxiphoid incision, which provides an entrance into the abdominal cavity, and by a second incision through the diaphragm and the pericardium to provide entry from the abdominal cavity to the inferior aspect of the heart. Implantable cardiac pacemakers can be implanted using this approach. However, this technique does not provide sufficient exposure of the coronary arteries or heart valves to allow it to be used for coronary artery bypass surgery or cardiac valve replacement.
Some procedures may be performed using a suprasternal approach, during which a rigid endoscope is inserted into the mediastinal cavity (the cavity which contains the heart and the other organs positioned between the pleural sacs) through a small incision formed above the sternum. However, this technique is useful for a limited number of procedures, such as visualization of the mediastinal cavity (mediastinoscopy), and small procedures such as lymph node biopsy.
Access to the anterior extrapericardial space for closed cardiac massage in cardiac arrest may be gained using an intercostal approach. For this approach, an incision is formed in the space between the ribs adjacent the sternum. An inflatable ring is inserted through the incision and placed over the heart. The ring is inflated to compress the heart and to thereby circulate blood during cardiac arrest. Because the heart lies just beneath the sternum, very little working space exists for manipulation of the inflatable ring or for insertion of surgical instruments between the heart and the sternum and adjacent ribs.
Thoracoscopic techniques may allow access to the heart from a lateral approach, through the pleural cavity. Multiple incisions are formed in intercostal sites on one side of the chest. The lung on that side of the chest is collapsed, and the pleura is incised to give access to the heart in the mediastinum. Disadvantages of this approach include the increased instrument length required to reach the heart, and the lack of access to the anterior surface of the heart, which is covered by the sternum.
Since the subxiphoid, suprasternal, intercostal and thoracoscopic approaches provide limited access to the heart, researchers are presently considering endovascular substitutes for open heart surgery. For example, some researchers have reported animal studies in which a total endovascular approach was used to implant heart valves in piecemeal fashion in animals and to assemble the valves in situ. This approach has inherent technical difficulties; for example, the development of reliable valves capable of being assembled in situ. Even some conventional heart valves designed for open heart placement suffer from fatigue fracture problems.